CHARACTERS DELIGNE-LUSZTIG THEORY LUSZTIG’S JORDAN DECOMPOSITION R EPRESENTATION THEORY FOR GROUPS OF L IE TYPE LECTURE III: DELIGNE-LUSZTIG THEORY Gerhard Hiss Lehrstuhl D für Mathematik RWTH Aachen University HIM Summer School Logic and Algorithms in Group Theory September 12, 2018
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CHARACTERS DELIGNE-LUSZTIG THEORY LUSZTIG’S JORDAN DECOMPOSITION
REPRESENTATION THEORY FOR GROUPS OF LIE
TYPELECTURE III: DELIGNE-LUSZTIG THEORY
Gerhard Hiss
Lehrstuhl D für MathematikRWTH Aachen University
HIM Summer SchoolLogic and Algorithms in Group Theory
September 12, 2018
CHARACTERS DELIGNE-LUSZTIG THEORY LUSZTIG’S JORDAN DECOMPOSITION
CONTENTS
1. Characters2. Deligne-Lusztig theory3. Lusztig’s Jordan decomposition
CHARACTERS DELIGNE-LUSZTIG THEORY LUSZTIG’S JORDAN DECOMPOSITION
RECOLLECTION
Objective: Classify all irreducible representations of all finitesimple groups and related finite groups,
find labels for their irreducible representations, find the degreesof these, etc.In the following: G = GF a finite reductive group over F,char(F) = p,
k an algebraically closed field.Recall that we distinguish three cases:
1. char(k) = p; defining characteristic
2. char(k) = 0; ordinary representations
3. 0 < char(k) 6= p; non-defining characteristic
Today I mainly talk about Case 2, so assume that char(k) = 0for the time being.
CHARACTERS DELIGNE-LUSZTIG THEORY LUSZTIG’S JORDAN DECOMPOSITION
A SIMPLIFICATION: CHARACTERS
Let V , V ′ be kG-modules.
The character afforded by V is the map
χV : G→ k , g 7→ Trace(g|V ).
Characters are class functions.
V and V ′ are isomorphic, if and only if χV = χV ′ .
Irr(G) := {χV | V simple kG-module}: irreducible characters
C: set of representatives of the conjugacy classes of G
The square matrix[χ(g)]χ∈Irr(G),g∈C
is the ordinary character table of G.
CHARACTERS DELIGNE-LUSZTIG THEORY LUSZTIG’S JORDAN DECOMPOSITION
CHARACTERS DELIGNE-LUSZTIG THEORY LUSZTIG’S JORDAN DECOMPOSITION
GOALS AND RESULTS
Objective: Describe all ordinary character tables of all finitesimple groups and related finite groups.
Almost done:
1. For alternating groups: Frobenius, Schur2. For groups of Lie type: Green, Deligne, Lusztig, Shoji, . . .
(only “a few” character values missing)3. For sporadic groups and other “small” groups:
Atlas of Finite Groups, Conway, Curtis,Norton, Parker, Wilson, 1986
CHARACTERS DELIGNE-LUSZTIG THEORY LUSZTIG’S JORDAN DECOMPOSITION
THE GENERIC CHARACTER TABLE FOR SL2(q), q EVEN
C1 C2 C3(a) C4(b)
χ1 1 1 1 1
χ2 q 0 1 −1
χ3(m) q + 1 1 ζam + ζ−am 0
χ4(n) q − 1 −1 0 −ξbn − ξ−bn
a,m = 1, . . . , (q − 2)/2, b,n = 1, . . . ,q/2,
ζ := exp(2π√−1
q−1 ), ξ := exp(2π√−1
q+1 )[µa 00 µ−a
]∈ C3(a) (µ ∈ Fq a primitive (q − 1)th root of 1)[
νb 00 ν−b
]∈∼ C4(b) (ν ∈ Fq2 a primitive (q + 1)th root of 1)
Specialising q to 4, gives the character table of SL2(4) ∼= A5.
CHARACTERS DELIGNE-LUSZTIG THEORY LUSZTIG’S JORDAN DECOMPOSITION
DRINFELD’S EXAMPLE
The cuspidal simple kSL2(q)-modules have dimensions q − 1and (q − 1)/2 (the latter only occur if p is odd).
How to construct these?
Consider the affine curve
C = {(x , y) ∈ F2 | xyq − xqy = 1}.
G = SL2(q) acts on C by linear change of coordinates.
Hence G also acts on the étale cohomology group
H1c (C, Q̄`),
where ` is a prime different from p.
It turns out that the simple Q̄`G-submodules of H1c (C, Q̄`) are
the cuspidal ones (here k = Q̄`).
CHARACTERS DELIGNE-LUSZTIG THEORY LUSZTIG’S JORDAN DECOMPOSITION
DELIGNE-LUSZTIG VARIETIES
Put k := Q̄`.
Deligne and Lusztig (1976) construct for each pair (T, θ),where T is an F -stable maximal torus of G, and θ ∈ Irr(TF ), ageneralised character RG
T (θ) of G.(A generalised character of G is an element of Z[Irr(G)].)
Let (T, θ) be a pair as above.
Choose a Borel subgroup B = TU of G with Levi subgroup T.(In general B is not F -stable.)
Consider the Deligne-Lusztig variety associated to U,
YU = {g ∈ G | g−1F (g) ∈ U}.
This is an algebraic variety over F.
CHARACTERS DELIGNE-LUSZTIG THEORY LUSZTIG’S JORDAN DECOMPOSITION
DELIGNE-LUSZTIG GENERALISED CHARACTERS
The finite groups G = GF and T = TF act on YU, and theseactions commute.
Thus the étale cohomology group H ic(YU, Q̄`) is a
Q̄`G-module-Q̄`T ,
and so its θ-isotypic component H ic(YU, Q̄`)θ is a Q̄`G-module,
whose character is denoted by ch H ic(YU, Q̄`)θ.
Only finitely many of the vector spaces H ic(YU, Q̄`) are 6= 0.
Now put
RGT (θ) =
∑i
(−1)ich H ic(YU, Q̄`)θ.
This is a Deligne-Lusztig generalised character.
CHARACTERS DELIGNE-LUSZTIG THEORY LUSZTIG’S JORDAN DECOMPOSITION
PROPERTIES OF DELIGNE-LUSZTIG CHARACTERS
The above construction and the following facts are due toDeligne and Lusztig (1976).
FACTSLet (T, θ) be a pair as above. Then
1. RGT (θ) is independent of the choice of B containing T.
2. If θ is in general position, i.e. NG(T, θ)/T = {1}, then±RG
T (θ) is an irreducible character.
FACTS (CONTINUED)
3. For χ ∈ Irr(G), there is a pair (T, θ) such that χ occurs inRG
T (θ).
CHARACTERS DELIGNE-LUSZTIG THEORY LUSZTIG’S JORDAN DECOMPOSITION
A GENERALISATION
Instead of a torus T one can consider any F -stable Levisubgroup L of G.
Warning: L does in general not give rise to a Levi subgroup ofG in the sense of my first lecture.
Consider a parabolic subgroup P of G with Levi complement Land unipotent radical U, not necessarily F -stable.
The corresponding Deligne-Lusztig variety YU is defined asbefore: YU = {g ∈ G | g−1F (g) ∈ U}.
One gets a Lusztig-induction mapRG
L⊂P : Z[Irr(L)]→ Z[Irr(G)], µ→ RGL⊂P(µ).
CHARACTERS DELIGNE-LUSZTIG THEORY LUSZTIG’S JORDAN DECOMPOSITION
PROPERTIES OF LUSZTIG INDUCTION
The above construction and the following facts are due toLusztig (1976).
Let L be an F -stable Levi subgroup of G contained in theparabolic subgroup P, and let µ ∈ Z[Irr(L)].
FACTS
1. RGL⊂P(µ)(1) = ±[G : L]p′ · µ(1).
2. If P is F -stable, then RGL⊂P(µ) = RG
L (µ) is theHarish-Chandra induced character.
It is now (almost) known, that RGL⊂P is independent of P.
CHARACTERS DELIGNE-LUSZTIG THEORY LUSZTIG’S JORDAN DECOMPOSITION
UNIPOTENT CHARACTERS
DEFINITION (LUSZTIG)A character χ of G is called unipotent, if χ is irreducible, and if χoccurs in RG
T (1) for some F-stable maximal torus T of G,where 1 denotes the trivial character of T = TF .We write Irru(G) for the set of unipotent characters of G.
The above definition of unipotent characters uses étalecohomology groups.
So far, no elementary description known, except for GLn(q);see below.
Lusztig classified Irru(G) in all cases, independently of q.
Harish-Chandra induction preserves unipotent characters (i.e.Irru(G) is a union of Harish-Chandra series), so it suffices toconstruct the cuspidal unipotent characters.
CHARACTERS DELIGNE-LUSZTIG THEORY LUSZTIG’S JORDAN DECOMPOSITION
THE UNIPOTENT CHARACTERS OF GLn(q)
Let G = GLn(q) and T the torus of diagonal matrices.
Then Irru(G) = {χ ∈ Irr(G) | χ occurs in RGT (1)}.
Moreover, there is bijection
Pn ↔ Irru(G), λ↔ χλ,
where Pn denotes the set of partitions of n.
This bijection arises from EndkG(RGT (1)) ∼= Hk ,q(Sn) ∼= kSn.
The degrees of the unipotent characters are “polynomials in q”:
Known to be true for GLn(q) (Dipper-James, 1980s) and ifCG∗(s) is a Levi subgroup of G∗ (Bonnafé-Rouquier, 2003).The truth of this conjecture would reduce the computation ofIrr`(G) to unipotent characters.
CHARACTERS DELIGNE-LUSZTIG THEORY LUSZTIG’S JORDAN DECOMPOSITION
GENERICITY
Recall that |G| = qN ∏mi=1 Φi(q)ai , where Φi is the i th
cyclotomic polynomial and ai ∈ N.
If ` | |G| but ` - q, there is a smallest e ≤ m with ae > 0 suchthat ` | Φe(q).
CONJECTURE (GECK)If ` is not too small, there is a natural bijection Irru(G)↔ Irru
` (G).
In particular, Irru` (G) can be classified independently of q.
CONJECTURE (GENERICITY CONJECTURE)If ` is not too small, the values of the element of Irru
` (G) onlydepend on e, not on `.
This would reduce the computation of Irru` (G) to finitely many
cases: finitely many e ≤ m, finitely many small primes `.
CHARACTERS DELIGNE-LUSZTIG THEORY LUSZTIG’S JORDAN DECOMPOSITION